The combined application of organic manure and chemical fertilizers is an effective way to enhance soil organic carbon (SOC) sequestration through its influences on organic carbon (OC) input and the stability of SOC fractions. However, there is limited information on the carbon sequestration efficiency (CSE) of chemically separated SOC fractions and its response to OC input under long-term fertilization regimes, especially at different sites. This study used three long-term fertilization experiments in Gongzhuling, Zhengzhou and Qiyang spanning 20 years to compare the stocks and CSE in four different OC fractions (very labile OC, labile OC, less labile OC, and non-labile OC) and their relationships with annual OC input. Three treatments of no fertilization (CK), chemical nitrogen, phosphorous, and potassium fertilizers (NPK), and chemical NPK combined with manure (NPKM) were employed. The results showed that compared with CK, NPKM resulted in enhanced SOC stocks and sequestration rates as well as CSE levels of all fractions irrespective of experimental site. Specifically for the very labile and non-labile OC fractions, NPKM significantly increased the SOC stocks by 43 and 83%, 77 and 86%, and 73 and 82% in Gongzhuling, Qiyang, and Zhengzhou relative to CK, respectively. However, the greatest changes in SOC stock relative to the initial value were associated with non-labile OC fractions in Gongzhuling, Zhengzhou, and Qiyang, which reached 6.65, 7.16, and 7.35 Mg ha–1 under NPKM. Similarly, the highest CSE was noted for non-labile OC fractions under NPKM followed sequentially by the very labile OC, labile OC, and less-labile OC fractions, however a CSE of 8.56% in the non-labile OC fraction for Gongzhuling was higher than the values of 6.10 and 4.61% in Zhengzhou and Qiyang, respectively. In addition, the CSE for the passive pool (very labile+labile OC fractions) was higher than the active pool (less-labile+non-labile OC fractions), with the highest value in Gongzhuling. The redundancy analysis revealed that the CSEs of fractions and pools were negatively influenced by annual OC input, mean annual precipitation and temperature, but positively influenced by the initial SOC and total nitrogen contents. This suggests that differential stability of sequestered OC is further governed by indigenous site characteristics and variable amounts of annual OC input.

The rapid development of the economy and society has led to a rapid expansion of the volume of territorial and resource data in China, which has brought certain difficulties to the development of the land engineering industry. However, ArcGIS software not only overcomes many inconveniences in land data statistics, but also can be applied to all aspects of land use change. It can modify, organize and output the final change statistics summary table of the huge volume of territorial data in land engineering one by one. The operation is simple and the query is convenient. This article analyzes the application of ArcGIS software in the analysis of unused land reserve resources, the production of standard sheet maps, and the superimposition of surveying and mapping and land use status maps in land engineering one by one, and points out that the development prospects of ArcGIS software in the field of land engineering are huge, ultimately achieving the goal of efficient utilization and development of land resources in China.

Stabilization of aeolian sand is essential for achieving desertification control, soil and water conservation, and agricultural development in sandy lands. Feldspathic sandstone is a soft clay rock widely found in the Mu Us Sandy Land. The purpose of this study was to ascertain the mechanism for aeolian sand stabilization with feldspathic sandstone from the perspective of particle size distribution. Feldspathic sandstone was added to aeolian sand at different ratios (mf:ms = 1:0, 1:1, 1:2, 1:5, and 0:1, where mf is the mass of feldspathic sandstone and ms is the mass of aeolian sand). The results showed that the soil texture was modified upon addition of feldspathic sandstone. The content of particles <0.05 mm increased with increasing addition ratio of feldspathic sandstone, in contrast to the downward trend observed for particles >0.05 mm. Consequently, the soil texture changed from sand to sandy loam, then loam, and finally silty loam. The addition of feldspathic sandstone ameliorated aeolian sand, resulting in a broader particle size distribution and lower particle size uniformity. Continuously well-graded soil was obtained at mf:ms = 1:5 (coefficient of uniformity: 54.71; coefficient of curvature: 2.54) or 1:2 (coefficient of uniformity: 76.21; coefficient of curvature: 1.12). While the addition of feldspathic sandstone solved the problem of single particle size distribution in aeolian sand, the presence of aeolian sand prevented soil compaction caused by the high clay content of feldspathic sandstone. Findings of this study indicate that the addition of feldspathic sandstone to aeolian sand leads to the mixing of various sized particles and continuous gradation of the soil. Although a higher addition ratio of feldspathic sandstone is more favorable for soil texture improvement, mf:ms = 1:5 is recommended for practical application in terms of particle gradation and cost effectiveness.

The stoichiometry of efficient soil microbial carbon use is a sensitive index for measuring changes in soil quality and plays a crucial role in research on ecological stoichiometry in the soil nutrient cycle. To further understand the effect of feldspathic sandstone and sand compound ratios on microbial carbon use efficiency (CUE), we simulated the field conditions of the feldspathic sandstone-sand compound layer in the Mu Us sandy land and analyzed the soil C:N:P ratio, microbial biomass, extracellular enzyme activity, and microbial carbon use efficiency in soils with different compound ratios. The results demonstrated that an increase in the feldspathic sandstone content had insignificant effects on the soil C:N:P ratio. The maximum values for microbial biomass nitrogen (MBN) and microbial biomass phosphorus (MBP) were observed at compound ratios of 1:5 and 1:2, respectively. Calculations of microbial carbon use efficiency and vector analysis revealed that the microbial carbon use efficiency increased as the feldspathic sandstone content increased, P limitation existed in all compound soils, and soil with a 1:1 compound ratio may be substantially less limited. In conclusion, our research indicated that adding feldspathic sandstone to sand improved soil quality, and the compound ratio affected soil microorganisms; nevertheless, it did not significantly change soil nutrient restriction. Our study provides a theoretical basis for the development and utilization of desert land resources.

Agricultural production in the Weibei rainfed highland, Northwest China, is challenged by severe drought and water shortages. While the land use pattern has shifted gradually from crop production to orchard farming in Weibei, little is known about the influence of fruit industry development on regional water resources and the rationality of planting orchards. Here, we characterized soil water depletion patterns in rainfed orchards and farmlands to evaluate the occurrence of soil desiccation under land use conversion from farmlands to orchards in Weibei. Soil moisture dynamics were monitored in the 0–150 cm soil profiles of different aged Red Fuji apple orchards (young: 7 years, mature: 13 years, old: 22 years) and long-term cultivated winter wheat fields. We measured soil moisture content by oven-drying method in the middle of each month during the growing season of apple trees (March–September 2019). The over-depletion and depletion of soil water were analyzed to evaluate water stress and differential water depletion by distinct vegetation, respectively. The soil desiccation index was used to determine the occurrence of dry soil layers. Water stress was only observed at the 0–70-cm soil depths in the old orchards (mid-June) and farmlands (mid-May–mid-July). Water depletion took place at deeper depths for longer periods in the older orchards than in the younger orchards. Soil desiccation was absent in the young orchards, with mild desiccation at the 0–80-cm soil depths in the mature and old orchards in mid-June. The desiccation intensity was mild at the 0–60-cm soil depths in mid-April–mid-May, intense at the 0–150-cm soil depths in mid-June, and moderate at the 20–150-cm soil depths in mid-July. Results of this study demonstrate the mitigation of water stress and soil desiccation following conversion from wheat fields to apple orchards, which verifies the rationality of planting orchards in the rainfed highland area. Our findings provide strong support for developing a novel model of agro-industrial development, ecological construction, and sustainable economy in the vast arid and semi-arid areas of Northwest China.

Agricultural production in Weibei rain-fed highland, Northwest China, is facing severe drought and water shortages. Here, soil water consumption characteristics in rain-fed orchards and farmlands were explored to ascertain the rationality of planting orchards in Weibei. Soil moisture dynamics was monitored in the 0–150-cm soil profiles of different aged ‘Red Fuji’ apple orchards (young: 7 years, mature: 13 years, and old: 22 years), and in long-term cultivated winter wheat fields during the growing season of apple trees. The over-consumption and consumption of soil water were analyzed to evaluate water stress and differential water consumption by distinct vegetation, respectively. Soil desiccation index was used to determine the occurrence of dry soil layers. Generally, there was no water stress in the 0–150-cm orchard soil profiles, while water stress was observed at the 0–70-cm soil depths in the old orchards (mid-June) and farmlands (mid-May–mid-July). Water consumption took place at deeper depths for longer periods in the older orchards than in the younger orchards. Soil desiccation was not observed in the young orchards, while mild desiccation occurred at the 0–80-cm soil depths in the mature and old orchards in mid-June. The desiccation intensity was mild at the 0–60-cm soil depths in mid-April–mid-May, intense at the soil 0–150-cm depths in mid-June, and moderate at the 20–150-cm soil depths in mid-July. In conclusion, conversion from wheat fields to apple orchards could reduce soil water stress, reduce dry soil layers, and mitigate soil desiccation in the rain-fed highland area.

Soil moisture is very important for the growth of fruit trees. Nutrients are dissolved in water, and the flow of water drives the transport of solutes, so that they can be absorbed and utilized by crops.

The soil microbial community plays an important role in regulating soil organic carbon (SOC) decomposition and maintaining stability in forest ecosystems. However, the interactions between the soil microbial community and soil carbon (C) fractions following afforestation remain poorly understood. In this study, soil samples were collected in an afforested area representing a chronosequence of 42, 27 and 17 years of Robinia pseudoacacia L. succession (RP42yr, RP27yr and RP17yr, respectively), and in farmland (FL) soil for comparison. Illumina sequencing of the16S rRNA and fungal ITS genes was used to analyse soil bacterial and fungal diversity, and the content of C fractions was also measured. Our results indicated that soil C fractions in the afforested RP42yr, RP27yr and RP17yr sites were 34.83–94.11%, 38.52–82.83% and 27.24–89.32% larger, respectively, than in the FL soil. Shannon indices for bacterial and fungal diversity, which ranged from 6.59 to 6.81 and 3.73 to 4.19, respectively, were also larger in the afforested soil. In addition, the dominant bacterial and fungal phyla, including Proteobacteria , Acidobacteria , Chloroflexi , Gemmatimonadetes , Bacteroidetes , Nitrospirae , Verrucomicrobia , Planctomycetes , Armatimonadetes , Cyanobacteria , Chlorobi , Firmicutes , Fibrobacteres , Zygomycota , Basidiomycota and Glomeromycota , were more abundant in afforested soil than in FL soil, whereas Actinobacteria and Ascomycota were more abundant in FL soil than in afforested soil. Spearman's rank correlation coefficients were strong and positive between soil microbial diversity (alpha diversity, Bshannon and Fshannon) and C fractions ( P < 0.05). The dominant phyla (both bacterial and fungal), such as Proteobacteria and Zygomycota , had significant positive effects on C fractions, whereas for other taxa, such as Actinobacteria and Ascomycota , they were significant and negative. Thus, our results indicated that changes in soil C fractions are linked to the composition of soil microbial communities following afforestation. They also provide further evidence that soil bacterial and fungal communities play vital roles in the turnover of SOC and C cycling. Highlights Changes in soil microbial community influence soil carbon fractions following afforestation. Carbon fractions and soil microbial communities respond to afforestation. Changes in C fractions were strongly correlated with soil microbial diversity. Soil microbial diversity and microbial taxa markedly affected carbon fractions.

Chlorophyll, as an important factor for the normal growth and development of plants, is of great significance for the management of agricultural water and fertilizer. In this study, the chlorophyll content of maize leaves was taken as the research object, and the chlorophyll content prediction model was quantitatively studied. ASD FieldSpec Pro spectrometer was used to measure the spectral reflectance of the leaf samples, and the spectral curve characteristics of different contents were analyzed. The results show that: (1) The reflection spectrum undergoes the nine-point smoothing of Savitzky-Golay and combines with the MSC, NOR, and SNV transforms to significantly increase the signal-to-noise ratio of the reflection spectrum. The combination band with higher correlation can significantly improve the stability and prediction of the model ability. (2) In the PLSR model, MSC processing is performed on the smoothed spectrum, and the model established after the second-order differential transformation has the best effect, Rc 2 = 0.95, RMSEC = 2.32, SEC = 1.35.

Effects of different inorganic and organic amendments on soil quality after land consolidation of hollow villages were investigated to provide evidence for the efficient use and rational land consolidation of abandoned, vacant homesteads in rural China. A plot experiment was conducted using raw soil collected from the hollow village land consolidation project area in Chengcheng County, Shaanxi Province, China. The soil was filled into 0 - 30 cm depth of the plots and mixed with different inorganic and organic amendments: non-amended soil was treated as the control. After one season of summer maize cultivation in the plots, soil water, nutrient, and pH were examined and compared among the four treatments. Following summer maize cultivation, average water content and total water storage in the 0 - 100 cm soil profile were highest in the FS-CM treatment, exceeding the control group by 48.0 and 60.2%, respectively (p values < 0.05). Similar trends were found for soil total nitrogen, available phosphorus, available potassium, and organic matter contents, which were 32.2, 154.0, 14.8 and 85.8%, respectively higher in FS-CM compared with the control (all p values < 0.05). Soil water, available phosphorus, and organic matter levels were significantly higher in FS-CM than in CM, while soil water, total nitrogen, and available potassium levels were increased in CM compared with the control (p values < 0.05). Soil pH was significantly reduced from 7.59 - 5.86 by CM and to 6.72 by FS-CM (p value < 0.05). The effects of FS on the soil properties tested were minimum among the three soil amendment treatments. After land consolidation of hollow villages, addition of different amendments increased both water and nutrient levels and modified pH conditions in the soil, thereby improving its overall quality. Combining chicken manure and ferrous sulfate appears to be the optimal strategy for soil improvement rather than applying either alone.

Microbial biomass, extracellular enzyme activity, and their stoichiometry in soil play an important role in ecosystem dynamics and functioning. To better understand the improvement of sand soil quality and the limitation of soil nutrients after adding feldspathic sandstone, we investigated changes in soil microbial activity after 10 months of mixing feldspathic sandstone and sand, and compared the dynamics with soil properties. We used fumigation extraction to determine soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and microplate fluorometric techniques to measure soil β-1,4-glucosidase (BG), β-1,4-xylosidase (BX), β-D-cellobiohydrolase (CBH), N-acetyl-β-glucosaminidase (NAG), and Alkaline phosphatase (AKP). We also measured soil organic carbon (SOC), pH, electrical conductivity (EC), soil inorganic carbon (SIC), and soil water content (SWC). Our results showed that the soil microbial biomass C, N, P, and individual extracellular enzyme activities significantly increased in mixed soil. Similarly, the soil microbial biomass C:N, C:P, N:P, MBC:SOC, and BG:NAG significantly increased by 54.3%, 106.3%, 33.1%, 23.0%, and 65.4%, respectively. However, BG:AKP and NAG:AKP decreased by 19.0% and 50.3%, respectively. Additionally, redundancy analysis (RDA) and Pearson’s correlation analysis showed that SWC, SOC, porosity and field capacity were significantly associated with soil microbial biomass indices (i.e., C, N, P, C:N, C:P, N:P in microbial biomass, and MBC:SOC) and extracellular enzyme activity metrics (i.e., individual enzyme activity, ecoenzymatic stoichiometry, and vector characteristics of enzyme activity), while pH, EC, and SIC had no correlation with these indices and metrics. These results indicated that mixing feldspathic sandstone and sand is highly susceptible to changes in soil microbial activity, and the soil N limitation decreased while P became more limited. In summary, our research showed that adding feldspathic sandstone into sand can significantly improve soil quality and provide a theoretical basis for the development of desertified land resources.

Effects of feldspathic sandstone as soil amendment on mechanical and mineral composition, water storage and yield of corn (Zea mays L.) were investigated. The experiment was carried out in a field containing sandy soil in Mu Us Sandy Land of China in between 2012 and 2014. In the first year of the experiment, treatments included four rates of feldspathic sandstone to sandy soil in volume. After feldspathic sandstone and sandy soil compounding, the texture characteristics of compound soil changed gradually from sand to silt loam and the content of montmorillonite, goeschwitzite and kaolinite gradually increased. The water storage of the surface soil (30 cm) showed an increasing trend of polynomial function with y = 8.286 x 0.705 (y for water storage, %, x for four kinds of treatments, x = 1, 2, 3, 4). Grain yield of the corn crop increased from 67.8-160.1%. The treatment with a rate of 1 : 1 (feldspathic sandstone to sandy soil in volume) had the great effect on soil physical properties averaged over the three years. The treatment with rate of 1 : 2 had the greatest effect on crop performance. Feldspathic sandstone showed great prospect for improving soil physical properties and crop yield in Mu Us Sand Land and deserve further study.

Chlorophyll is an important factor for measuring the normal growth and development status of plants, and it is also of great significance for the management and utilization of agricultural water and fertilizers. In this study, the chlorophyll content of rapeseed leaves was taken as the research object, and the effect of spectral data pretreatment method on the spectral feature extraction and chlorophyll content prediction model was quantitatively studied. ASD FieldSpec Pro (350-2500 nm) spectrometer was used to measure the spectral reflectance of rape leaf samples, and the spectral reflectance characteristics of different chlorophyll contents were analyzed. The Savitzky-Golay nine-point smoothing of the reflectance spectrum was performed, and the first derivation (FD), second derivation (SD), and reciprocal logarithm (LOG) transformation of the reflectance were performed after MSC and SNV preprocessing respectively. The optimal spectral estimation model for chlorophyll was established by PLSR. The results show that: (1) This study was mainly to monitor the chlorophyll content in rape joints during jointing stage, using the correlation between chlorophyll content and hyperspectral characteristics, using MSC, NOR and SNV to pretreat the reflectance spectra and combining different derivations transformations to extract chlorophyll characteristics. (2) Quantitative model of chlorophyll content was established based PLSR, the best preprocessing was R + SG + SNV + LOG+ FD, the calibration results was: LVs = 14, Rc(2) = 0.97, RMSEC = 4.18, SEC = 4.21, Slope = 0.92, Offset = 2.63; the validation results was: Rv(2) = 0.98, RPD = 7.52, RMSEP = 2.94, SEP = 2.98, Slope = 0.98, Offset = 1.43; (3) The optimal estimation model established by different treatment methods has better stability and higher precision, and can rapidly monitor the chlorophyll content of rapeseed in the region. (C) 2019 Elsevier B.V. All rights reserved.

The purpose of this study is to ecologically soil, reduce fertilization and maintain land productivity. In this study, the amount of nutrient elements stored in the soil is large, and the soil samples are treated with long-term constant and different water content to determine the activation amount of the solid elements in the soil. The results showed that the content of available phosphorus and available potassium released by XH6 in different water content cultures was higher than 3.89% and 10.12% of XH10, respectively. The content of nitrate nitrogen released by XH4 was higher than that of treated XH4. %. The content of ammonium released under treatment of XH10 was higher than that of treatment with XH10 of 24.11%. The release nutrients varied with the culture time, but the release of nutrients such as ammonium nitrogen, nitrate nitrogen, available phosphorus and available potassium peaked on the 14th day of culture, and stabilized in the latter culture. So, the high moisture content has a certain release and increase effect on soil available potassium and available phosphorus.

The Mu Us sandy land in China’s Shaanxi Province faces a critical water shortage, with its aeolian sandy soil endangering the regional eco-environment. Here we investigated the effects of feldspathic sandstone on water retention in an aeolian sandy soil from the Mu Us sandy land. Feldspathic sandstone and aeolian sandy soil samples were mixed at different mass ratios of 0:1 (control), 1:5 (T1), 1:2 (T2), and 1:1 (T3). Soil-water characteristic curves were determined over low- to medium-suction (1–1000 kPa) and high-suction (1000–140 000 kPa) ranges, by centrifuge and water vapor equilibrium methods, respectively. Results showed that the addition of feldspathic sandstone modified the loose structure of the aeolian sandy soil mainly consisting of sand grains. The van Genuchten model described well the soil-water characteristic curves of all four experimental soils (R2-values > 0.97). Soil water content by treatment was ranked as T2 > T3 > T1 > control at the same low matric suction (1–5 kPa), but this shifted to T2 > T1 > T3 > control at the same medium- to high-suction (5–140 000 kPa). T2 soil had the largest saturated water content, with a relatively high water supply capacity. This soil (T2) also had the largest field capacity, total available water content, and permanent wilting coefficient, which were respectively 17.82%, 11.64%, and 23.11% higher than those of the control (P-values < 0.05). In conclusion, adding the feldspathic sandstone in an appropriate proportion (e.g., 33%) can considerably improve the water retention capacity of aeolian sandy soil in the study area.

Land engineering as a new discipline has been temporarily outrageous. The proposition of soil body organic reorganization undoubtedly enriches the research content for the construction of land engineering disciplines. Soil body organic reconstruction is designed to study how to realize the ecological ecology of the land by studying the external force of nature, to study the influence of sunlight, wind and water on soil body, how to improve the soil physical structure, to further strengthen the research of biological enzymes and microbes, and promote the release and utilization of beneficial inert elements in soil body. The emerging of frontier scientific research issues with soil body organic reorganization to indicate directions for the future development of soil engineering.

Soil aggregation is one of the most important factors affecting soil organic carbon (SOC) stabilization, and the stability of aggregates depends in part on soil microbial diversity and composition. Interactions between the soil bacterial community and SOC content in soil aggregates after afforestation are poorly understood. In this study, we investigated difference in the diversity of soil bacterial with high-throughput 16S rRNA sequencing, as well as the SOC content in soil aggregates representing a chronosequence of 42, 27, and 17 years of Robinia pseudoacacia L. succession (RP42, RP27, and RP17), and in farmland (FL) soil for comparison (millet (Setaria italica) and soybean (Glycine max) rotation).The SOC content in RP17, RP27, and RP42 plots were significantly higher than that of FL by an average of 85.57%, 142.37%, and 76.69% in large macro-aggregates (>1 mm), small macro-aggregates (0.25–1 mm), and micro-aggregates (<0.25 mm), respectively. The Simpson index for the FL plot was significantly higher than that of the RP17, RP27, and RP42 plots, whereas the Shannon index followed the opposite trend. The dominant bacterial phyla detected were Proteobacteria, Acidobacteria, and Actinobacteria in each afforested and FL sites. These data revealed significant correlations between soil aggregate characteristics, such as SOC content, mean weight diameter (MWD), and geometric mean diameter (GMD), with the relative abundance of Proteobacteria, Actinobacteria, Acidobacteria, Chloroflexi, Gemmatimonadetes, Nitrospirae, Verrucomicrobia, and Planctomycetes. These relationships suggested that the effects of afforestation on SOC stabilization in soil aggregates are modulated by both soil aggregate size and also soil bacterial diversity. We demonstrate that the interaction between soil aggregate size and soil microbes might be a key factor in effective soil conservation, restoration, sustainability of agroecosystems, and erosion prevention.

Soil respiration (SR) is an important process in the carbon cycle. However, the means by which changes in understory plant community traits affect this ecosystem process is still poorly understood. In this study, plant species surveys were conducted and soil samples were collected from forests dominated by black locust (Robinia pseudoacacia L.), with a chronosequence of 15, 25, and 40 years (RP15, RP25, and RP40, respectively), and farmland (FL). Understory plant coverage, evenness, diversity, and richness were determined. We investigated soil microbial biomass carbon (MBC), nitrogen (MBN), phosphorus (MBP), and stoichiometry (MBC:MBN, MBC:MBP, and MBN:MBP). Soil enzyme assays (catalase, saccharase, urease, and alkaline phosphatase), heterotrophic respiration (HR), and autotrophic respiration (AR) were measured. The results showed that plant coverage, plant richness index (R), evenness, and Shannon-Wiener diversity were higher in RP25 and RP40 than in RP15. SR, HR, and AR were significantly higher in the forested sites than in farmland, especially for SR, which was on average 360.7%, 249.6%, and 248.2% higher in RP40, RP25, and RP15, respectively. Meanwhile, catalase, saccharase, urease, and alkaline phosphatase activities and soil microbial C, N, P, and its stoichiometry were also higher after afforestation. Moreover, significant Pearson linear correlations between understory plants (coverage, evenness, diversity, and richness) and SR, HR, and AR were observed, with the strongest correlation observed between plant coverage and SR. This correlation largely depended on soil enzymes (i.e., catalase, saccharase, urease, and alkaline phosphatase), and soil microbial biomass C, N, and P contents and its stoichiometry, particularly urease activity and the MBC:MBP ratio. Therefore, we conclude that plant communities are drivers of soil respiration, and that changes in soil respiration are associated with shifts in soil enzyme activities and nutrient stoichiometry.

Afforestation is recognized as an important driving force for soil organic C (SOC) dynamics and soil element cycling. To evaluate the relationships between soil C:N:P stoichiometry and SOC fractions, soil C:N:P stoichiometry distributions at 0–200 cm soil depths were analyzed and the contents of SOC fractions were evaluated in 9 typical land-use systems on the Loess Plateau of China. The contents of light fraction organic C, particulate organic C (> 53, 53–2 000, and > 2 000 µm), labile organic C, microbial biomass C, and dissolved organic C decreased with increasing soil depth and were higher in afforested soil than in slope cropland soil. Compared with the slope cropland, different vegetation types influenced soil C:N, C:P, and N:P ratios, especially when C:P and N:P ratios were significantly higher (P < 0.05). Moreover, SOC fractions at the 0–10 and 10–40 cm depths were particularly affected by soil C:P ratio, whereas those at the 40–100 and 100–200 cm soil depths were significantly affected (P < 0.05) by soil N:P ratio. These results indicate that changes in SOC fractions are largely driven by soil C:P and N:P ratios at different soil depths after afforestation.

Core Ideas Soil C fractions were significantly correlated with microbial C/P and N/P ratios. Soil C fractions were extremely affected by microbial C/P and N/P ratios. Microbial C, N, and P stoichiometry have the potential to affect C storage. Microbial C/N/P stoichiometry ratios are of interest because of important ecosystem fluxes of C, N, and P, such as mineralization and immobilization, and they can help to characterize the cycling of soil elements, especially regarding soil C sequestration. However, it is not clear how soil microbial C/N/P stoichiometry influences C cycling. In this study, the concentrations of microbial biomass C, microbial biomass N, microbial biomass P, and soil C fractions were measured in 45‐, 40‐, and 25‐yr‐old black locust (Robinia pseudoacacia L.) forest soils and a sloped cropland. The results showed that soil organic C (SOC) concentrations or stocks and the percentage increment of the SOC stocks were highest under 45‐yr‐old black locust (RP45a). The concentrations of each C fraction in the black locust forest soils (45a, 40a, and 25a) were higher than that of sloped cropland in the 0‐ to 30‐cm soil profile. The concentrations of microbial C, N, and P in RP45a were also higher by an average of 29.6 to 232.9, 9.2 to 17.9, and 0.6 to 1.1 mg kg−1, respectively, compared with the same concentrations in sloped cropland at the 0‐ to 30‐cm soil depth. Redundancy analysis indicated that soil C fractions, especially for particulate organic C (53–2000 and >2000 μm), were significantly correlated with the microbial C/P and N/P ratios, and the “best” model selection indicated that the microbial N/P and C/P ratios significantly changed with soil C fractions with regard to stand age and soil depth in the black locust forests. Therefore, the present study demonstrated that C fractions respond to microbial C, N, and P stoichiometry after farmland‐to‐forest conversion and hence have the potential to affect C storage in the loess hilly region.